Problem: Nadia did 27 more sit-ups than Christopher in the morning. Christopher did 1 sit-up. How many sit-ups did Nadia do?
Answer: Christopher did 1 sit-up, and Nadia did 27 more than that, so find the sum. The sum is $1 + 27$ sit-ups. She did $1 + 27 = 28$ sit-ups.